N-methyl-2-pyrrolidone (NMP) is a valuable solvent for paint strippers, graffiti removers, agricultural chemicals, floor polishes, automotive cleaners, industrial cleaners, and photoresist strippers. NMP is commonly manufactured by reacting methylamine with gamma-butyrolactone (see, e.g., U.S. Pat. No. 6,348,601). The most common way to purify NMP during the manufacturing process involves one or more distillation steps.
Even after manufacture, however, there is often a need to improve the quality of NMP, particularly for applications that require exceptionally high purity, such as electronics and pharmaceutical applications. Traces of amine impurities can remain in the NMP even after thorough distillation. Moreover, NMP can develop undesirable color upon prolonged storage, particularly if the solvent contains traces of acids, amines, or other impurities.
Amine-contaminated NMP has been purified with gel-type and macroporous cation exchangers (see U.S. Pat. Nos. 5,777,131 and 6,207,824). Basic ion-exchange resins have been used to remove acidic impurities from NMP when the NMP was used as an extraction solvent and required purification thereafter (see U.S. Pat. No. 6,217,771). U.S. Pat. No. 4,501,902 describes another process for removing acidic impurities from contaminated NMP with a solid inorganic compound, which could be “alumina.” A water-washed, activated alumina was used to purify NMP after the NMP was used to extract aromatic compounds from lubricating oil distillates (see U.S. Pat. No. 4,837,338).
In sum, aluminas have only infrequently been proposed for purifying NMP, and even then, they have generally been used to purify NMP that has been contaminated with impurities from a washing or extraction process. Little is known about the potential benefits of purifying NMP immediately post-manufacture with aluminas. Even less is known about how to select aluminas that will afford the greatest benefit in eliminating amine impurities and reducing color.